1. Field of the Invention
The present invention relates to an automatic adjustment circuit for automatically adjusting a central frequency of an oscillator.
2. Description of the Prior Art
For example, a voltage controlled oscillator (hereinafter referred to as VCO), which is used in various electronic appliances, is often formed with its central frequency shifted from a predetermined value, because of variations in characteristics of individual circuit elements. It has therefore been customary to cope with such a shift by controlling a constant of a capacitor or a coil, or by manipulating a potentiometer.
However, this is far from eliminating manual adjustments and, accordingly, it does not lead to efficient production. Moreover, in the case where an oscillator has to be integrated in an IC in order to reduce the number of externally mounted parts, it is not feasible to adjust oscillation frequency by controlling a constant of a coil, because some circuit elements such as a coil cannot be connected to an IC.
On the other hand, there are electronic appliances in which exact adjustment of a central frequency of an oscillator is essential. For example, a hi-fi VHS-format VCR (videocassette recorder) is counted as such. A VCR of this type requires two carriers for recording and reproducing a sound signal in each of different television systems, that is, 1.3 MHz and 1.7 MHz in the NTSC system, 1.4 MHz and 1.8 MHz in the PAL system, and so on. When these two carriers are modulated with a sound signal, each of them will have an extension of .+-.150 kHZ at the maximum, leaving a margin of only 100 kHz between the two FM signals. In addition, there exist a luminance signal and a chrominance signal in the frequency bands just above and below that for a sound signal, and consequently, there remains only a range of about 10 kHz as a permissible range for a deviation of the sound signal carrier. In this case, it is unrealizably difficult to adjust a control input voltage to a VCO by manipulation of a potentiometer by a production worker.
Conventionally, a circuit as shown in FIG. 1 has been proposed for automatic adjustment of an oscillator. Such a conventional circuit is found in a hi-fi sound circuit in a 8-mm format VCR, for example. This circuit is constructed as a PLL (Phase Locked Loop) circuit. An output of a current controlled oscillator (CCO) 90 is divided by a frequency divider 91 and delivered to a phase comparator 94. The phase comparator compares phases between this signal and a reference frequency, which is separately fed in from a terminal 92 and divided by a frequency divider 93. The comparison output is smoothed by a low-pass filter 95, converted into a current by a voltage/current converter 96, amplified by a current amplifier 97, and then supplied as a control signal to the CCO 90. In the drawing, fo represents a signal oscillated by the CCO.
However, the conventional circuit is defective in the following respects. A reference frequency f.sub.sc is required always to be inputted. Since the circuit may synchronize with noises if it receives any, it necessitates a provision for preventing noises. Moreover, it consumes power to keep a PLL constantly in action. Further, being an analog circuit, it requires a large circuit scale. In addition, if there is more than one oscillator, each oscillator has to be provided with a PLL circuit, because one PLL circuit can control only one oscillator.